The present invention relates to a method of forming a ferroelectric thin film of vinylidene fluoride homopolymer.
Polymer type ferroelectric materials have advantages such as flexibility, light weight, good processability and low price as compared with inorganic ferroelectric materials such as ceramics. There are known, as represented examples thereof, vinylidene fluoride polymers such as polyvinylidene fluoride (PVdF) and vinylidene fluoride/trifluoroethylene (VdF/TrFE) copolymer.
With respect to PVdF, crystal structures thereof are roughly classified into three kinds such as I-form (also said to be β-form), II-form (α-form) and III-form (γ-form). Among them, it is only I-form crystal that can exhibit sufficiently high ferroelectricity.
PVdF having a high molecular weight which is prepared by radical polymerization method forms crystal form II and does not exhibit ferroelectricity as it is. In order to convert the crystal form II of PVdF to crystal form I, there are required complicated post-steps such as stretching and heat-treating of a film or rapid cooling under high pressure at casting.
Matsushige et al have studied formation of thin film of vinylidene fluoride oligomer having crystal form I by using vinylidene fluoride oligomer: CF3(CH2CF2)nI (number average degree of polymerization n=17) having crystal form II, and in this study, have found that a thin film of vinylidene fluoride oligomer of crystal form I was formed only by deposition coating on a KBr substrate at a substrate temperature Ts>0° C., for example, at 25° C. Also it was found that in deposition-coating on a KCl substrate at 25° C., a coating film containing a mixture of I-form and II-form crystals was formed and was almost converted to a thin film of vinylidene fluoride oligomer of crystal form I by heat-treating (at a temperature of not less than 110° C.) after the coating. However, in a substrate (SiO2, Pt, Au, or the like) having small interaction with vinylidene fluoride oligomer, just after the deposition, and further even after the following heat treatment, only thin films containing a mixture of I- and II-form crystals have been obtained (M & BE Vol. 11, No. 2, 145 (2000)).
Also Matsushige et all have recently found that a thin film of vinylidene fluoride oligomer of crystal form I could be formed on various substrates by deposition of vinylidene fluoride oligomer of crystal form II under extremely low temperature environment of not more than −130° C. (Polymer Preprint, Japan, Vol. 51, No. 12, 3097 (2002)).
As mentioned above, thin films of I-form crystal have not been formed only by coating at room temperature except coating on KBr substrate.
Okui et all have made analysis of crystal structure with respect to vinylidene fluoride oligomer: CCl3(CH2CF2)nCl (number average degree of polymerization n=9) prepared by radical polymerization by using CCl4 as a chain transfer agent (telogen) and dinormalperoxy dicarbonate as a catalyst, and have reported that this oligomer was a mixture of crystal form I (β-form) and crystal form III (γ-form) and had a crystalline melting point Tm at two points (74° C. and 110° C.) (Polymer Journal, Vol. 30, No. 8, pp. 659 to 663 (1998), and POLYMER Vol. 38, No. 7, pp. 1677 to 1683 (1997)).
In this case, thin films were formed by casting or the like for the purpose of analyzing crystal structures. However, the thin films were formed simply in a process of structural analysis, and no studies have been made with respect to polarization intended to obtain ferroelectricity and resultant ferroelectric characteristics.
In the mentioned process for preparing vinylidene fluoride oligomer by using CCl4 as a chain transfer agent (telogen), a molecular weight distribution of the obtained vinylidene fluoride oligomer is wide, and even if crystal forms I (β-form) are obtained, the crystal structures easily become a mixture of I-form (β-form) with II-form (α-form) and III-form (γ-form) and a purity of crystal form I (β-form) becomes low, which lowers ferroelectric characteristics of thin films formed by using the obtained vinylidene fluoride oligomer even if polarization is carried out.
By the way, there are various known processes for preparing polymers using vinylidene fluoride monomer.
For example, “iodine transfer polymerization process” using perfluoroalkyl iodide as a chain transfer agent (or telogen) is known, and a molecular weight distribution of high molecular weight polymer can be made narrow particularly in non-crystalline polymers used for fluorine-containing rubbers or the like such as vinylidene fluoride/hexafluoropropene copolymer and vinylidene fluoride/tetrafluoroethylene/hexafluoropropene copolymer (Kobunshi Ronbunshu, 49(10), No. 10, pp. 765 to 783 (1992)).
In addition, there are known other polymerization processes such as a polymerization process not using a chain transfer agent and a polymerization process using a hydrocarbon chain transfer agent (telogen) such as isopentane or alcohol. However there is a problem that a molecular weight distribution of the obtained polymer becomes wide, and a purity of crystal form I (β-form) is lowered like the above-mentioned preparation processes.
Also with respect to polymerization processes for a low molecular weight vinylidene fluoride polymer, there are disclosed a process using, as a telogen, the same perfluoroalkyl iodide as above (JP56-57811A), a process using alcohols as a telogen (JP59-117503A) and a process using perfluoroalkyl bromide as a telogen (JP63-93736A, JP7-179523A, etc.).
However all of those preparation processes are intended to prepare vinylidene fluoride copolymers (copolymer oligomers), and studies on vinylidene fluoride homopolymers (homopolymer oligomers) having crystallinity have not been made. Furthermore there are disclosed no processes for efficiently preparing, at high purity, vinylidene fluoride homopolymers having crystal form I (β-form) which can exhibit ferroelectric characteristics.
As mentioned above, vinylidene fluoride homopolymer comprising crystal form I alone or as main component has not yet been obtained as a green powder product just after polymerization, and as a matter of course, there is no example of production of ferroelectric thin film using the green powder product of vinylidene fluoride homopolymer comprising crystal form I alone or as main component.